1. Field of the Invention
The present invention relates to a low thermal expansion Ni superalloy, and more particularly to a low thermal expansion Ni superalloy having high strength and excellent corrosion-resistance and oxidation-resistance.
2. Description of the Related Art
In recent years, the bolt material for high temperature use in a pressure vessel member which is heated to high temperature, such as a chamber of a steam turbine and gas turbine is made of 12 Cr ferritic steel (containing C: 0.12%, Si: 0.04%, Mn: 0.7%, P: 0.1%, Ni: 0.4%, Cr: 10.5%, Mo: 0.5%, Cu: 0.03%, V: 0.2%, W: 1.7%, Nb: 0.% and Fe: remaining percent) or austenitic heat-resistant alloy (Nimonic alloy 80A including Cr: 20.5%, Mn: 0.4%, Al: 1.4%, Ti: 2.4%, Si: 0.3%, C: 0.06 Zr: 0.06%, B: 0.003%, Ni: remaining percent, and Refrataloy 26 including Cr: 18%, Co: 20%, Mo: 3%, Ti: 2.6%, Fe: 16%, C: 0.05%, Ni: remaining percent).
In recent years, in order to improve the thermal efficiency of a steam turbine, the steam temperature has been further increased so that the high temperature bolt has been used under more severe conditions. Where each of the materials described above is used for the high temperature bolt under such a severe condition, 12 Cr ferritic steel is low in cost and excellent in production. However, if the steam temperature becomes higher than at present, the material is low in strength at the high temperature. On the other hand, austenitic heat-resistant alloy is better in the corrosion-resistance and oxidation-resistance than the 12 Cr ferritic steel, and high in the high temperature strength. However, because it has a higher linear expansion coefficient than that of 12 Cr ferritic steel, it may produce leakage of steam due to insufficient tightening of the bolt, and generate thermal fatigue. Therefore, austenitic heat-resistance alloy is also problematic as a material used at higher temperatures.
JP-A-9-157779 discloses a low thermal expansion Ni-base super heat-resistant alloy with excellent corrosion-resistance and oxidation-resistance containing, by weight %, C of 0.2% or less, Si of 1% or less, Mn of 1% or less, Cr of 10 to 24%, one or more kinds of Mo and W of Mo+½ W of 5 to 17%, Al of 0.5 to 2%, Ti of 1 to 3%, Fe of 10% or less, B of 0.02 or less and Zr of 0.2% or less, and as necessary Co of 5% or less and Nb of 1.0% or less and remainder of Ni and inevitable impurities. JP-A-8-85838 also discloses a similar alloy.
A previously known example of alloys having a low linear expansion coefficient is Inconel 783 of an Invar alloy (containing Cr: 3.21%, Mn: 0.08%, Al: 5.4%, Ti: 0.2%, Si: 0.07%, C: 0.03%, B: 0.003%, Fe: 24.5%, Ni: 28.2% and Co: 35.3% . . . Comparative Example No. 2) which has been developed as the material for a jet engine. This alloy has a low linear expansion coefficient in a ferromagnetic state with the Curie point adjusted in the balance of Fe—Ni—Co. However, this alloy does not have enough corrosion-resistance to be used for the steam turbine.